Over-statement.
"You are what you read" kinda implies that the content of a book defines you.
Problem is, that logic can take you to weird places.
Reading a romantic book doesn't make you a romantic anymore than reading a (modern) book where the protagonist is an unrepentant racist make you a KKK member.

This whole thread is precisely about the myths that try to link specific reading genres to stereotypical cliches; "LitFic readers are snobbish brie and wine, liberal arts graduates incapable of programming a DVR!" being just one. "Or, comic book readers are nerds and geeks incapable of even talking to a member of the opposite sex."

You may or not have meant it that way but that is what "you are what you read" leads to.

Some of us are eclectic enough in our reading that nothing we read comes even close to defining us in any way. And even if all a person reads is from one genre, that still doesn't tell you anything about their personality, IQ, social skills, or grooming habits, other than the fact that they like to read that particular genre for unfathomable reasons.

Humans are complicated and their habits and rationale for doing anything tends to be irrational so they can't be summarized in short-n-snappy sentences. Trying to do so tends to annoy many of them, too.